The present invention relates to a shear foil and to its method of manufacture and more particularly to such foils which are employed in electrically-operated dry shavers.
Shear foils are constituted with a plurality of apertures and are generally mounted in electric shaver heads and also serve to protect the face of a user from accidentally coming into contact with the sharp cutting edges of the cutting mechanism located in the interior of the shaver. The shear foil is also provided at its side facing the cutting mechanism with counteredges which project in direction towards the cutting mechanism and which cooperate with the aforementioned cutting edges of the latter so as to cut any hair follicles which pass through the apertures intermediate the cutting edges and the counteredges. These respective cooperating edges move relative to each other and are exposed thereby to a high degree of abrassive wear.
It is known in the prior art to manufacture shear foils by coating both sides of a thin metal plate with a photosensitive chrome film and to print a grid pattern on one of these sides. This latter side is then exposed and developed by washing to remove parts of chrome layer. Thereupon an etching bath etches the plate at those portions thereof which are not covered with the film. The etching process is continued until the entire thickness of the metal plate is completely eroded, i.e. until the other coated side is reached. Now, hard material is electrolytically-deposited at the exposed side portions of the metal plate. After removing all of the remaining film on both sides of the plate the screen is comprised of elements whose projecting portions or legs are entirely constituted of the electrolytically-deposited hard material.
As hard material, those skilled in the art generally use metals such as metal carbide which generally contains other metals as binding material. Moreover, one may use hard nickel or hard cobalt for electrodeposition; for example, hard nickel owes its hardness characteristic to impurities, such as sulfur normally contained therein. In the art of shear foils, hard nickel is almost exclusively used. However, such material is unfortunately prone to react and thereby to corrode with atomic hydrogen which is normally generated in the electro-deposition plating baths. The larger the surface to be plated, the stronger is the corrosion effect caused by virtue of the hydrogen presence.
Thus, since the prior art suffers from the drawback of having to build up the entire height of the projecting legs of the screen, it will be appreciated that the rather lengthy time involved in forming such legs and the rather large surface area involved all combine to multiply the deleterious effect caused by the hydrogen. The counteredges thus produced are not sharp, not strong, cannot maintain an edge, are subject to rapid wear, are not long lasting, and are easily prone to breakage.
It is also known in the art of electric shavers to improve cutting efficiency by coating the cutting edges themselves with a hard substance such as diamond, silicon carbide, aluminum oxide, titanium carbide, titanium boride, or titanium nitride. Alloys based on nickel, cobalt or copper are also used. The coating thickness generally lies between 3 and 50 microns, the portion of hard metal material actually used constituting between 5 and 80% by volume of the cutter blades.
It is also known to coat the entire rear surface of the screen with hard metal and to build up a substantial thickness of such hard metal over the entire area of the screen so that it can be later machine-ground away to form the desired cutting edges. As examples of hard metals actually used, it is known to use hard metals comprised of 17% by volume of diamond powder which has an average particle size of 1 micron, and to electrolytically deposit nickel as an additional metal in the proportion of 83% by volume to a thickness of approximately 6 microns. Such prior art approaches suffer from the additional expenses of using expensive extra material, extra machining operations, and the necessity of having to plate over an area of substantial size, as well as the increased deleterious effect generated by the greater presence of hydrogen. Moreover, the additional 6 micron layer of nickel which is used only as a binder does not have acceptable wear-resistance characteristics in the presence of the hydrogen. Very frequently, lubricants must be provided intermediate the cutting edges of the cutting mechanism and the screen foil so as to provide a longer working life for the shaver.